The invention relates to communication. More particularly, the present invention relates to a method to select a physical layer transmission rate or physical rate in a wireless communication system.
Wireless network is considered as the preferred network for home environment. The multimedia applications over wireless networks impose new challenges in networking efficient utilization of network bandwidth and computing resource where quality of service (QoS) requirement of video and audio is met.
The wireless network is composed per example of an access Points (AP) and a lot of station (STA) such as per example telephone, radio, Visio phone, television, computer, movie player. In this communication system signals are linked through a channel from a Station or an Access Point to a Station or an Access Point.
Wireless Multimedia Extensions (WME), also known as WiFi Multimedia, WMM) is a WiFi Alliance interoperability certification based on the IEEE 802.11e draft standard. It provides basic Quality of service (QoS) features to IEEE 802.11 networks. The traffic specification is defined in the 802.11e standard. It contains parameters that define the characteristic and the QoS expectation of traffic flow.
The first generation of WiFi Network i.e. 802.11b, was oriented for transport of best effort traffic i.e. mainly TCP transport.
The objective of the rate control algorithm (RCA) implemented within the station (STA) or access point (AP) is to provide the maximum of bandwidth: this implies to minimize the CUE (Channel Usage Estimation) used by each AP (Access Point) or STA (Station).
As highest phyrate (physical rate) usage decrease the transmission time but increase also the number of retransmission, the generation of algorithm tries to find the best compromise between the phyrate value and the “Retry Rate” value by methods specific to each algorithm. The “Retry Rate” is the way to measure retransmission, it is defined by the ratio for a population of N packet by: (Total number of retransmissions/N).
The difficulty comes from the fact that there are two origins of the retransmission difficulties:                The degradation of wireless condition increasing the BER (Bit Error Rate)        The collisions with packet coming from other APs and STAs. Trials show that the collision ratio is much more above the simulation ratio, due to mainly the performance and response time of wireless channel detection.        
Unfortunately, at HW (Hardware level) level there is no practical way to identify the origin of a retransmission difficulty.
Consequently, there is also no way also to measure independently the Retry Rate due to collision (called Collision Retry Rate) and retry rate due to BER (called BER Retry Rate).
Furthermore, the collision detection is not possible for WiFi NWs
In order to optimize the transport over RTP (transport protocol) of real time media like Voice, interactive or non-interactive video, and associated signaling, the IEEE/WiFi Alliance has defined prioritization mechanism within WMM (WiFi manufacturer mode) which defines a contention channel access based on AC (Access Category). It has introduced four AC (Access Category) to improve the QoS on the wireless link: BK (Back ground) category, BE (Best Effort) Category, VI (Video) Category, VO (Voice) Category.
The WMM prioritization mechanism does not guaranty any value of QoS performance parameters; however trials show that taking account of WMM prioritization improves de facto, lot of the global behavior.
In the reality, the collision avoidance prioritization provided by the standard is limited:
The priority between AC is not absolute but relative: the collision ratio of each traffic flow is the result of collision probability with all other traffic flow reduced (i.e. multiply) by the collision probability of collision provided by the WMM collision avoidance mechanism.                The collision avoidance mechanism only work when several STAs or APs are waiting for a free channel. When the channel is already free, there is no collision avoidance mechanism between concurrent access.        The TXOP (Transmit Opportunity) mechanism increases the bandwidth performance by sending several packets during the same medium access, so it reduces the concurrent access probability but also increase the waiting delay for free medium.        
Consequently, the collision avoidance mechanism works better for continuous transmission (typically bulk transfer) than sporadic or periodic transmission like voice or video media.
In the document “Bit-rate selection in wireless networks” from J. C. Bicket, an evaluation of bit-rate selection techniques to maximize throughput over wireless link is described. But this selection does not take account of WMM prioritization mechanism.
The existing Bandwidth oriented RCA algorithms providing a common phyrate to all ACs are functional in WMM environment, but are not adapted for this environment.